A liquid crystal display uses optical and electrical properties of a liquid crystal material to display images. Such a liquid crystal display has some advantages, including light weight, low power consumption and low driving voltage, as compared to cathode ray tubes (CRT), plasma display panels, or the like. A liquid crystal display includes a liquid crystal layer disposed between one glass substrate and another glass substrate. The light emitted from a light source passes through the liquid crystal layer, which controls the transmission of light. The light that has passed through the liquid crystal passes through a color filter layer, and the light that has passed through the color filter layer is used to realize a full-color screen by an additive mixture of colors.
In general, as methods for producing a color filter for use in a liquid crystal display, there are known a dyeing method, printing method, electrodeposition method and a pigment dispersion method. Although methods using a dye have been discussed to date, use of a dye causes problems of lower heat resistance, photoresistance and chemical resistance as compared to a pigment, and thus shows limited applicability. In addition, in the case of a dyeing method, a complicated process is required, resulting in poor cost efficiency. Therefore, recently, a pigment dispersion method has been used generally. Although a pigment has lower transparency than a dye, such a disadvantage has been overcome by advanced technology of micronization and dispersion of a pigment. Since a color filter obtained by a pigment dispersion method uses a pigment, it is stable against light, heat, solvent, or the like. When such a color filter is subjected to patterning through photolithography, it is easy to produce a color filter for use in a large-screen and high-precision color display. Thus, such a color filter is now used most widely.
A pigment used for a pigment dispersion type color resist includes each of a red, green and a blue pigments, when forming an RGB color filter. In general, the pigment may further include a yellow pigment, purple pigment, or the like in order to represent colors more effectively. A method for producing a color filter by pigment dispersion includes applying a color resist solution onto a substrate by using a spin coater, followed by drying to form a coating film. Then, the coating film is subjected to pattern exposure and development to obtain colored pixels, and heat treatment is carried out at high temperature to obtain a first color pattern. This process is repeated according to the number of colors, thereby providing a color filter. The most important factor determining the quality of a color resist is the properties of a pigment used as a colorant, dispersibility thereof and dispersion state thereof. Recently, as LCDs have been produced to have a large scale and high precision, requirements for a color filter, such as high transmission of a colored layer, high contrast, reduced width of a black matrix and high reliability, have been increased every year. As a means for satisfying such requirements, a pigment is micronized to the highest degree in order to meet color properties, such as luminance and contrast, according to the related art.
However, the pigment powder obtained from its synthesis cannot be used as it is, because preparation of a pigment dispersion requires a stable dispersion state and easy micronization. Thus, a micronization process, such as salt milling, is required, but such a post-treatment process is not preferred in terms of environmental protection. Moreover, a number of additives, such as a dispersing agent and pigment derivative, are required, and a very complicated and cumbersome process is required to obtain a pigment dispersion. In addition, such a pigment dispersion requires complicated storage and transport conditions to maintain its optimal quality state.
In the case of a pigment dispersion, the pigment is present as particles and thus causes light scattering. In addition, micronization of a pigment causes a rapid increase in surface area of pigment, degradation of dispersion stability and generation of non-uniform pigment particles. For these reasons, it is difficult to produce a color resist.
In addition, many studies have been conducted recently to accomplish high luminance, high contrast and high resolution by using a pigment. However, the above-mentioned problems related with pigment micronization and dispersion stability lead to insufficient improvement of physical properties of a color filter using a pigment as a colorant.
As a substitute for a pigment dispersion, some studies have been conducted to improve luminance and contrast by using a hybrid type colorant containing a pigment in combination with a dye to improve physical properties. Although such studies lead to slight improvement of luminance and contrast, they are merely adding some dyes to a pigment. Thus, it is not possible to improve physical properties significantly.
To solve the above-mentioned problems and to accomplish high luminance, high contrast and high resolution, some studies have been conducted more recently about using a dye instead of a pigment as a colorant. Particularly, many attempts have been made to use a triarylmethane dye as a blue colorant. In general, a triarylmethane dye has high transmission to 420-450 nm of a color filter, and thus shows excellent color characteristics as a blue colorant for a color filter. However, such a triarylmethane dye has low solubility to a solvent used for a colored composition for a color filter and shows poor heat resistance. In general, as a solvent for a colored composition for a color filter, propylene glycol monomethyl ether acetate (PGMEA), propylene glycol monomethyl ether (PGME) or cyclohexanone is used widely. In the case of a currently known dye, it is easy to ensure the solubility to cyclohexanone. However, such a dye has low solubility to PGMEA and PGME. Particularly, there is a tendency to prohibit use of cyclohexanone since it is an environmentally harmful material. Thus, there is a need for a dye having high solubility to PGMEA of PGME.
As one example of some studies about triarylmethane dyes according to the related art, there have been disclosed a salt compound having a cation of triarylmethane dye with an anion of naphthalenesulfonate or naphtylaminesulfonate that shows improved solubility and heat resistance, a colored resin composition including the same, and a color filter using the same. However, such a compound has low solubility to propylene glycol monomethyl ether acetate (PGMEA) and shows poor heat resistance.
As another example, there has been conducted a study about a compound of a triarylmethane cation with an anion of another dye. Although such a compound provides partially improved heat resistance, it still has low solubility to an organic ester solvent, such as PGMEA.